The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
An aircraft is powered by one or more propulsive assembly each comprising a turbojet engine housed in a tubular nacelle. Each propulsive assembly is fastened to the aircraft by a pylon generally located under or on an airfoil or at the fuselage.
“Upstream” means what comes before a considered point or element, in the direction of the air flow in a turbojet engine, and “downstream” means what comes after the considered point or element, in the direction of the air flow in the turbojet engine.
A nacelle generally has a structure comprising an air intake upstream of the turbojet engine, a mid-section intended to surround a fan or the compressors of the turbojet engine and its casing, a downstream section able to house thrust reversal means and intended to surround the gas generator of the turbojet engine, and is generally ended by an ejection nozzle whose outlet is located downstream of the turbojet engine.
Conventionally, the space comprised between the nacelle and the turbojet engine is called secondary flow path.
Generally, the turbojet engine comprises a set of blades (compressor and optionally a fan or non-streamlined propeller) rotationally driven by a gas generator through a set of transmission means.
A lubricant distribution system is provided to provide a good lubrication of the transmission means and of any other accessories such as electrical generators, and to cool them.
During the flight, depending on the temperature and humidity conditions, ice may be formed on the nacelle, particularly at the external surface of the air intake lip equipping the air intake section.
The presence of ice or rime changes the aerodynamic properties of the air intake and disturbs the air conveying towards the fan. In addition, the rime formation on the air intake of the nacelle and the ice ingestion by the engine in case of detachment of ice blocks can damage the engine or the airfoil, and present a risk to the safety of the flight.
A solution to de-ice the external surface of the nacelle consists in preventing the formation of ice on this external surface while keeping the concerned surface at a sufficient temperature.
Thus, the lubricant heat can be used to heat the external surfaces of the nacelle, the lubricant being thereby cooled and able to be reused in the lubrication circuit.
Documents U.S. Pat. No. 4,782,658 and EP 1479889 particularly, describe the implementation of such de-icing systems using the engine lubricant heat.
More particularly, Document U.S. Pat. No. 4,782,658 describes a de-icing system using outside air bled by a scoop and heated through an air/oil exchanger to serve the de-icing. Such system allows a better control of exchanged heat energy, but the presence of scoops in the external surface of the nacelle results in a loss of aerodynamic performances.
Document EP1479889 describes, meanwhile a system for de-icing an air intake structure of a turbojet engine nacelle using an air/oil exchanger in a closed circuit, the heated inside air of the air intake structure being put into forced circulation by a fan.
It should be noted that the air intake structure is hollow and forms a closed chamber for the circulation of de-icing air heated by the exchanger disposed within this chamber.
Thus, the heat energy available for the de-icing depends on the lubricant temperature.
In addition, the exchange surface of the air intake structure is stationary and limited and the actually dissipated energy depends mainly on the heat required for the de-icing and then on the outer conditions.
It follows that the cooling of the lubricant, as well as the temperature at which the air intake is kept, are difficult to control.
There is another solution in which are associated a heat exchanger and conduits for the circulation of a fluid to be heated so as to form a plurality of loops for the recirculation of the fluid to be heated through the exchanger, and such that a circulation area of the fluid to be heated is in contact with an external wall so as to enable a heat exchange by conduction with the outside air in the nacelle. The circulation of the fluid to be heated is performed by forced circulation.
There are solutions to de-ice the turbojet engine nacelles by means of hot air bleeding. These solutions conventionally rely on a hot air bleeding in the compressor of the turbojet engine. This bled hot air is under high pressure and high temperature, for one hand it is fed directly into an air intake lip of a nacelle to be de-iced, for the other hand it is led to an air/air exchanger (i.e., precooler) where it is cooled by the outside air to be used for the cabin air conditioning and the de-icing of the aircraft airfoil.
It has been noticed that systems as previously presented for de-icing the air intake lip by cooling of the lubricant cause friction losses in the secondary flow path due to the presence of the exchanger, and engine thrust losses when an air bleeding is performed in the secondary flow path where these losses have a significant impact on consumption (they represent about 0.5% of the total consumption), but also that such systems have a poor efficiency when the turbojet engine runs at idle and/or at low power (for example during the taxiing phase of the aircraft or when the aircraft is descending) in the case where the cooling of the engine oil involves a bleeding of the air coming from outside of the nacelle.
Solutions consisting of de-icing the air intake lip by bleeding the hot air in the compressor have drawbacks particularly in that the high temperature of the bleed air in the compressor of the turbojet engine leads to the use of costly materials for the front bulkhead of the air intake to be de-iced and for the inlet piping with commonly more than a wall to reduce the risks of bursting, and that they implement a specific air bleeding on the high-pressure compressor which reduces the power or the available thrust of the turbojet engine. Indeed, the solutions for de-icing by hot air bleeding in the compressor of the turbojet engine presented hereinabove implement conventionally three air bleedings in the compressor including one dedicated for the de-icing of the air intake lip of the nacelle.